1. Field of the Invention
The invention relates to methods and apparatus for avoiding problems associated with extended use of electrochemical devices, namely degradation that can occur as a result of cycling the electrochemical device on and off.
2. Background of the Related Art
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity, it finds wide application in disinfection processes and the removal of cyanides, phenols, iron, manganese, and detergents. Thus, ozone has widespread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. Furthermore, the relatively short half-life of ozone makes it difficult to distribute so it is generally produced on-site and usually very near the point of use. However, the cost of generating equipment, and poor energy efficiency of production has deterred its use in many applications and in many locations.
Because ozone has a very short life in the gaseous form, and an even shorter life when dissolved in water, it is preferably generated in close proximity to where the ozone will be consumed. Traditionally it is generated at a rate that is substantially equal to the rate of consumption since conventional generation systems do not lend themselves to ozone storage. Ozone may be stored as a compressed gas, but when generated using corona systems the pressure of the output gas stream is essentially at atmospheric pressure. Therefore, additional hardware for compression of the gas is required, which in itself reduces the ozone concentration through thermal degradation. Ozone may also be dissolved in liquids such as water but this process generally requires additional equipment to introduce the ozone gas into the liquid, and at atmospheric pressure and ambient temperature only a small amount of ozone may be dissolved in water.
Because so many of the present applications for ozone only have the need for relatively small amounts of ozone, it is generally not cost effective to use conventional ozone generation systems such as corona discharge. Furthermore, since many applications require the ozone to be delivered under pressure or dissolved in water, as for disinfection, sterilization, treatment of contaminants, etc., the additional support equipment required to compress and/or dissolve the ozone into the water stream further increases system cost.
Electrochemical cells in which a chemical reaction is forced by added electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions that produce or consume electrons. These reactions take pace at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electrons transfer electric charge between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. Internal charge transfer is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short-circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
Unfortunately, electrochemical ozone generators, especially those having lead dioxide as the anodic electrocatalyst, experience a performance degradation that gets worse with successive shutdowns of the generator or cell. This degradation manifests itself as an increasing voltage requirement of the cell. In some applications, this degradation can be avoided by providing a battery backup system that maintains a trickle current to the cell. In U.S. Pat. No. 5,529,683, Critz teaches that this problem can also be avoid by applying a reverse potential to the cell during shutdown. While these approaches to the problem may be sufficient in some applications, they both presume a continuing supply of electrical current.
Therefore, there is a need for an ozone generator system that operates efficiently on standard AC or DC electricity and water to deliver a reliable stream of ozone gas that is generated under pressure for direct use by the application. It would be desirable if the system was self-contained, self-controlled and required very little maintenance. It would be further desirable if the system had a minimum number of wearing components, a minimal control system, and be compatible with low voltage power sources such as solar cell arrays, vehicle electrical systems, or battery power. Finally, it would be desirable if the electrochemical cell were designed to overcome the cycling limitations inherent to existing electrochemical ozone generators without requiring the continued use of electrical current. It would be even more desirable if the electrochemical cell were designed to avoid or reduce other lifetime limiting effects, such as impure water.
The present invention provides an ozone generating system that combines single-use elements or segments with an extended use fixture that is used to activate the single-use elements. One embodiment of the invention consists of a strip of proton exchange membrane (PEM) having the ozone producing catalyst applied directly onto one side of membrane. Optionally, the application of this catalyst may be divided into segments or patches, wherein each segment represents the limited-use portion of the ozone generator. Each segment may be advanced into a fixture that provides the balance of the electrochemical system required for operation of the ozone generator. This balance of system may include additional subsystems, with a power supply, water source, electrical contacts, electronic controllers, sensors and feedback components, being typical examples. After an individual segment is advanced into the operating fixture, the membrane may be hydrated by a water source and electrical contact made to the positive (anode) face of the membrane having the ozone generating catalyst and to the negative (cathode) side of the membrane which may also include a catalyst layer.
After water and electrical contacts are provided to the limited-use segment, the system now forms the basic elements of an electrochemical cell that may be used for electrolysis. With the application of electrical current, the system will begin electrolyzing the available water to generate ozone which may then be utilized. The operation of the generator can then continue until the performance degrades to unacceptable levels or until the source of ozone is no longer required. At that time the electrical power may be shut off or the electrical contacts physically removed from the limited-use element. When the limited-use element has reached or neared its operating lifetime, the used segment may be removed from the fixture and a new segment advanced into position. In this manner, the process can continue with the limited lifetime components of the electrolyzer being completely replaced in a simple and potentially automated manner.
The concept of the limited-use element may be extended to include all the elements necessary for operation of the ozone generator that undergo degradation or consumption. While not intended to be an exhaustive list, these degradable or consumable elements may include the anodic catalyst, cathodic catalyst, membrane, performance indicators, water supply, and electrical supply. It may also be advantageous to include aspects of the product handling system as limited-use elements, such as including a hydrophobic, gas permeable membrane over the anode so that ozone gas may pass directly into a process stream without introducing other fluids into the cell.